Human beings interact with computers through some type of interface. Among the most popular types of interfaces is a graphical user interface (GUI) that displays various interface elements on a visual display to be viewed by a user. Ideally, these interface elements are collected in some type of representative environment and each individual element is a graphical depiction of a real-world object that the user will quickly recognize and find intuitive. For example, a common paradigm for a representative user environment that may be presented on a visual display is the concept of a desktop with elements such as representations of dog-eared pieces of paper to represent documents.
To interact with the computer, the user typically selects an element with which to work. In order to do so, the user positions a pointer or cursor in the vicinity of the desired element. Positioning a pointer or cursor may be done using a positioning device such as a mouse, a trackball, a track pad, a pen and tablet, or a similar device in conjunction with a visual display such as a cathode ray tube (CRT) display or a liquid crystal diode (LCD) display. Alternatively, pressure-sensitive display screens, commonly called touch screens, may be employed as part of the overall user interface. Usually, when a touch screen is used, the user touches the screen at the approximate location of a graphical element to interact with that element as an alternative to using a mouse or other positioning device.
Several types of touch screens are common today. Each type has its own qualities and usually is pixellated. With a first type, called a capacitive system, the screen comprises several layers. A first layer capable of storing an electrical charge is placed on a second layer comprising a glass panel. When the user touches the monitor, a portion of the stored charge of the first layer is conducted from the first layer to the user's body. Therefore, the level of charge stored on the first layer decreases. Associated circuits measure this charge decrease and evaluate the relative differences in charge at each corner of the layer. From this information, the location of the touch event can be derived.
A second type of touch screen operates on a different principle. A surface acoustic wave system includes a pair of transducers arranged along a glass plate. A first transducer is placed along the x-axis of the plate and a second transducer is placed along the y-axis of the plate. One of the transducers operates as a sender and the other as a receiver. Additionally, reflectors are positioned on the glass plate to reflect a signal sent from one transducer to the other. The receiving transducer is able to determine whether a signal sent by the other transducer has been disturbed by a touch event. If so, the location of that touch event on the screen can be discerned.
A third type of touch screen is described as a resistive system. The resistive system incorporates multiple layers and includes a glass panel that is covered with conductive and a resistive metallic layers. Spacers separate the conductive layer and the resistive layer. Typically, a scratch-resistant layer is placed on top of the surface that will be touched by the user. In operation, electrical currents run through both the conductive layer and the resistive layer. When a user touches the screen, the resulting pressure from the touch forces the conductive layer and the resistive layer into contact. A change in the electrical field is detected and coordinates of the point of contact, typically with reference to a two-dimensional x-y coordinate system, such as a Cartesian coordinate system, are determined. Although all three of these touch screen types may be employed in conjunction with the systems and methods disclosed herein, either with or without modifications that will be apparent to ordinary skilled artisans, touch screens based upon resistive systems are particularly well-suited for use.
When using touch screens, the resolution available to detect touch events is commonly far lower than the resolution available to display graphics associated with the GUI. One approach that may be employed is to determine a pixel region on the screen that has been touched by the user and to calculate a relative center pixel that can be used as a single- or sub-pixel position estimate. Even though this approach is possible, there are still associated problems using the calculated position as the point with which a cursor or other pointer is associated. For example, a common pointer in GUI environments is depicted on screen as an arrow. Commonly, the pixel at the very tip of the arrow-shaped pointer is where the focus lies for tasks involving use of the pointer. This configuration allows for fine levels of control for interacting with elements of the GUI. However, when the user touches the touch screen with his finger, the user's finger may be large enough to partially or completely obscure the arrow-shaped pointer from the user's view. This problem is especially acute when using a touch screen to interact with GUI elements that are smaller than the size of the user's finger. Therefore, there is a need for computer users to be able to finely control the positioning of a cursor or pointer when using a touch screen providing relatively coarse levels of touch position detection.